According to equilibrium condensation calculations, corundum and hibonite are the earliest condensates from a cooling gas of solar composition (Kornacki and Fegley, 1984). Although it occurs only rarely on earth, hibonite (CaAl12O19) plays an important role in the study of meteorites. It is a common minor mineral in refractory
Ca-Al-rich inclusions (CAIs) from carbonaceous chondrites and a number of CAIs consist predominantly of hibonite, sometimes together with spinel and perovskite (Ireland, 1988; Hinton et al., 1988). Trace element and isotopic compositions of hibonite-bearing inclusions indicate a variety of formational processes, including condensation, distillation and crystallization from a melt; often multi-stage processes are required. Inclusions containing corundum as a major phase are much rarer. Two from Murchison, BB-5 and GR-1 (Bar-Mathews et al., 1982; MacPherson et al., 1984), consist predominantly of corundum and hibonite with minor perovskite. In addition, corundum grains from Murchison have been chemically separated and analyzed by ion microprobe (Virag et al., 1991). In order to better understand the role of hibonite (and its relationship to corundum) in refractory inclusions, we have investigated the evaporation behavior of this mineral.
Small fragments of a single crystal of Madagascar hibonite were selected and suspended from Re wire loops for evaporation in a vacuum furnace. In addition to Ca and Al, the hibonite contains small amounts of Si, Fe and Mg, as well as 8.5 wt.% TiO2 and
3.5 wt.% REE (Curien et al., 1956); it is also enriched in Th and U (Fahey et al., 1987). Samples were evaporated at either 2100°C or 2500°C for times between 5 and 120 minutes; mass losses range from 37.5 to 99.5 %. The residues consist of varying amounts of corundum, thorianite, an unknown Ca-Al-bearing phase that is strongly enriched in the REE, and a glass matrix. In residues
evaporated at 2100°C, the proportions of these minerals increase, relative to the amount of glass, with increasing mass loss; in places, corundum crystals are separated only by thin stringers of thorianite and the REE phase. At 2500°C, corundum reacts with the melt and its proportion decreases with increasing mass loss; it is completely absent from the most refractory residue.
The glass is MgO-, FeO- and SiO2-free in all residues; TiO2 is also below EMP detection. In residues with low degrees of mass loss, it is compositionally similar to hibonite, but contains several percent REE and Th, in addition to Ca and Al. With increasing mass loss, Ca and Al concentrations decrease and REE and Th concentrations increase. Glass in the most refractory residue contains
20.5 wt.% (REE)2O3 and 4.7 wt.% ThO2, but only 0.3 wt.% CaO. The composition of the REE phase also becomes more REE-rich and Ca-poor with increasing mass loss; it contains 64.0 wt.% (REE)2O3 and 1.3 wt.% CaO in the most refractory residue. Most refractory trace elements, including the REE, are uniformly enriched by up to a factor of 100 in the residues relative to the starting material; there is also no significant fractionation of the REE between individual phases. However, Ce, V, U and, in the more fractionated residues, Eu show volatility-related depletions that result from the dynamics of the evaporation process. Similar depletions have been observed in residues from the evaporation of the Allende CV chondrite (Floss et al., 1996). Ca isotopic compositions of the residues show enrichments in the heavy isotopes of up to 55 /amu, consistent with a Rayleigh distillation process.
Evaporation of hibonite produces corundum-bearing residues, as expected from phase equilibria considerations (Nurse et al., 1965). Thus, in principle, corundum-bearing inclusions may have a distillation origin. Hibonite from both BB-5 and GR-1 has lower Mg and Ti contents than most meteoritic hibonites and is, in this respect, similar to hibonite produced by evaporation of Allende (Floss et al., 1996); in the present experiments, glass from low mass loss residues is compositionally similar to these hibonites. Furthermore, (MacPherson et al., 1984) have suggested that GR-1, which has a hibonite center surrounded by corundum in turn surrounded by a hibonite mantle, formed by partial melting of the core hibonite and evaporation of the resulting liquid. However, both corundum-bearing inclusions have isotopically light Mg and Ca (Hinton et al., 1988), inconsistent with an origin through evaporation. More promising candidates as evaporation residues may be the Group 2 corundum grains of (Virag et al., 1991). These have low Ti and V, and mass fractionated oxygen isotopic compositions. Some also appear to have low Ce/La ratios. The scarcity of corundum-bearing inclusions, however, suggests that melting of hibonite, with or without concomitant evaporation, was a rare process in the solar nebula.
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